The invention relates to a process for purifying feed streams comprising aromatics in polymerization or alkylation processes by bringing them into contact with zeolites, which comprises passing the feed stream over at least two zeolites 1 and 2, with zeolite 1 having a mean pore size of from 0.3 to 0.5 nm and zeolite 2 having a mean pore size of from 0.6 to 0.8 nm.
Alkylated aromatics are obtained predominantly by catalytic alkylation of aromatics by means of olefins: ethylbenzene, for example, by alkylation of benzene by means of ethylene, cumene, for example, by alkylation of benzene by means of propylene. Catalysts used in the liquid phase are aluminum chloride and catalysts used in the gas phase are Lewis acids or synthetic zeolites. Zeolites are highly active catalysts both for alkylation and for transalkylation. Since the zeolite catalysts are susceptible to water, sulfur and other catalyst poisons, they lose their activity over time and have to be regenerated periodically.
Various processes have been proposed for lengthening the life of zeolite catalysts for alkylation reactions. WO 98/07673 describes the alkylation of benzene by means of, for example, propylene. The benzene is pretreated by passing it over aluminum oxides, silicates, aluminum silicates or acidic zeolites such as mordenites.
WO 00/35836 describes a method which comprises firstly alkylating an aromatic, purifying the alkyl aromatic obtained by means of a molecular sieve and finally reacting the purified alkyl aromatic with another aromatic in a transalkylation reaction to give a monoalkylaromatic. Suitable molecular sieves are suitable zeolites and mixtures thereof, with acidic zeolites such as MCM-22 being preferred. However, it has now been found that the effectiveness of these acidic zeolites is unsatisfactory.
WO 01/07383 discloses the purification of feed streams comprising olefins in polymerization or alkylation processes by passing the olefin, e.g. ethylene, over an adsorption bed of carbon black, activated carbon, aluminum oxides, silicates, aluminum silicates, various zeolites or molecular sieves. It is stated that the benzene or alkyl benzene feed stream is advantageously passed over an appropriate adsorption bed comprising the adsorbents mentioned; further details are not given.
The purifying action of these processes for aromatics is not satisfactory in all cases, or expensive adsorbents are required.